Method and apparatus for transmitting sounding reference signal in wireless access system supporting machine type communication

ABSTRACT

The present invention provides methods for transmitting an SRS in a wireless access system supporting machine type communication and apparatuses for supporting the same. A method for transmitting a sounding reference signal (SRS) by a terminal in a wireless access system supporting machine type communication (MTC) according to an embodiment of the present invention may comprise the steps of: receiving an SRS transmission parameter configured for repeated transmission of an SRS from a base station; and repeatedly transmitting an SRS during a predetermined SRS repeated transmission interval according to the SRS transmission parameter.

TECHNICAL FIELD

The present invention relates to a wireless access system supportingmachine type communication (MTC), and more particularly, to a method foran MTC user equipment to transmit a sounding reference signal (SRS) andapparatus for supporting the same.

BACKGROUND ART

Wireless access systems have been widely deployed to provide varioustypes of communication services such as voice or data. In general, awireless access system is a multiple access system that supportscommunication of multiple users by sharing available system resources (abandwidth, transmission power, etc.) among them. For example, multipleaccess systems include a Code Division Multiple Access (CDMA) system, aFrequency Division Multiple Access (FDMA) system, a Time DivisionMultiple Access (TDMA) system, an Orthogonal Frequency Division MultipleAccess (OFDMA) system, and a Single Carrier Frequency Division MultipleAccess (SC-FDMA) system.

DISCLOSURE OF THE INVENTION Technical Task

The present invention relates to a method of transmitting a soundingreference signal (SRS) in a wireless communication environmentsupporting MTC and apparatus for supporting the same.

A technical task of the present invention is to provide an SRSconfiguration method and an SRS transmission method for repeated SRStransmission in an MTC environment.

Another technical task of the present invention is to provideapparatuses for supporting the above-mentioned methods.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

Technical Solutions

The present invention provides methods of transmitting SRS in a wirelessaccess system supporting MTC and apparatuses for supporting the same.

In a first technical aspect of the present invention, provided herein isa method of transmitting an SRS (sounding reference signal) by a UE(user equipment) in a wireless access system supporting MTC (machinetype communication), including: receiving an SRS transmission parameterconfigured for repeated SRS transmission from an eNB (evolved Node B);and performing the repeated SRS transmission during a prescribed SRSrepeat transmission interval according to the SRS transmissionparameter. If the SRS repeat transmission interval overlaps withsubframes in which transmission of uplink control information isperformed, the repeated SRS transmission may not be performed in theoverlapping subframes.

In a second technical aspect of the present invention, provided hereinis a UE (user equipment) for transmitting an SRS (sounding referencesignal) in a wireless access system supporting MTC (machine typecommunication), including: a receiver; a transmitted; and a processorfor supporting the SRS transmission. In this case, the processor may beconfigured to control the receiver to receive an SRS transmissionparameter configured for repeated SRS transmission from an eNB (evolvedNode B) and control the transmitter to perform the repeated SRStransmission during a prescribed SRS repeat transmission intervalaccording to the SRS transmission parameter. If the SRS repeattransmission interval overlaps with subframes in which transmission ofuplink control information is performed, the repeated SRS transmissionmay not be performed in the overlapping subframes.

In this case, the repeated transmission of the SRS may be periodicallyperformed according to a prescribed SRS transmission period.Alternatively, the repeated transmission of the SRS may be aperiodicallyperformed only if there is a request from the eNB.

Moreover, the repeated transmission of the SRS may be performed in onlycell-specific SRS subframes. At this time, the UE may further receiveindications of subframes in which the repeated SRS transmission will beperformed among the cell-specific SRS subframes from the eNB. In thiscase, the repeated transmission of the SRS is performed in only theindicated subframes.

Furthermore, the SRS transmission parameter may include a parameter forgenerating an SRS sequence for the repeated SRS transmission and the SRSparameter may be configured such that the SRS sequence has an identicalsequence during the prescribed SRS repeat transmission interval.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

As is apparent from the above description, the embodiments of thepresent invention have the following effects.

First of all, by receiving repeatedly transmitted SRS, an eNB canestimate an uplink channel for an MTC user equipment placed in poorconditions more reliably.

Secondly, an uplink channel for an MTC user equipment can be efficientlyused through an SRS generation method and an SRS transmission method forrepeated SRS transmission, which are unique to the MTC user equipment.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 is a conceptual diagram illustrating physical channels used inthe embodiments and a signal transmission method using the physicalchannels.

FIG. 2 is a diagram illustrating a structure of a radio frame for use inthe embodiments.

FIG. 3 is a diagram illustrating an example of a resource grid of adownlink slot according to the embodiments.

FIG. 4 is a diagram illustrating a structure of an uplink subframeaccording to the embodiments.

FIG. 5 is a diagram illustrating a structure of a downlink subframeaccording to the embodiments.

FIG. 6 is a diagram illustrating an example of a component carrier (CC)and carrier aggregation (CA) used in an LTE_A system.

FIG. 7 illustrates a subframe structure of an LTE-A system according tocross-carrier scheduling.

FIG. 8 is a conceptual diagram illustrating a construction of servingcells according to cross-carrier scheduling.

FIG. 9 is a diagram illustrating one of SRS transmission methods used inthe embodiments of the present invention.

FIG. 10 (a) is a diagram illustrating a concept of periodic SRStransmission and FIG. 10 (b) is a diagram illustrating a concept ofaperiodic SRS transmission.

FIG. 11 is a diagram illustrating one example of a method for an MTCuser equipment to repeatedly transmit SRS in case that trigger type of aSRS transmission scheme is set to 0.

FIG. 12 is a diagram illustrating one example of a method for an MTCuser equipment to repeatedly transmit SRS in case that trigger type of aSRS transmission scheme is set to 1.

FIG. 13 is a block diagram of apparatuses for implementing the methodsdescribed in FIGS. 1 to 12.

BEST MODE FOR INVENTION

The embodiments of the present invention, which will be described indetail below, relate to methods of transmitting SRS in a wireless accesssystem supporting MTC and apparatuses for supporting the same.

The embodiments of the present disclosure described below arecombinations of elements and features of the present disclosure inspecific forms. The elements or features may be considered selectiveunless otherwise mentioned. Each element or feature may be practicedwithout being combined with other elements or features. Further, anembodiment of the present disclosure may be constructed by combiningparts of the elements and/or features. Operation orders described inembodiments of the present disclosure may be rearranged. Someconstructions or elements of any one embodiment may be included inanother embodiment and may be replaced with corresponding constructionsor features of another embodiment.

In the description of the attached drawings, a detailed description ofknown procedures or steps of the present disclosure will be avoided lestit should obscure the subject matter of the present disclosure. Inaddition, procedures or steps that could be understood to those skilledin the art will not be described either.

Throughout the specification, when a certain portion “includes” or“comprises” a certain component, this indicates that other componentsare not excluded and may be further included unless otherwise noted. Theterms “unit”, “-or/er” and “module” described in the specificationindicate a unit for processing at least one function or operation, whichmay be implemented by hardware, software or a combination thereof. Inaddition, the terms “a or an”, “one”, “the” etc. may include a singularrepresentation and a plural representation in the context of the presentinvention (more particularly, in the context of the following claims)unless indicated otherwise in the specification or unless contextclearly indicates otherwise.

In the embodiments of the present disclosure, a description is mainlymade of a data transmission and reception relationship between a BaseStation (BS) and a User Equipment (UE). A BS refers to a terminal nodeof a network, which directly communicates with a UE. A specificoperation described as being performed by the BS may be performed by anupper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality ofnetwork nodes including a BS, various operations performed forcommunication with a UE may be performed by the BS, or network nodesother than the BS. The term ‘BS’ may be replaced with a fixed station, aNode B, an evolved Node B (eNode B or eNB), an Advanced Base Station(ABS), an access point, etc.

In the embodiments of the present disclosure, the term terminal may bereplaced with a UE, a Mobile Station (MS), a Subscriber Station (SS), aMobile Subscriber Station (MSS), a mobile terminal, an Advanced MobileStation (AMS), etc.

A transmitter is a fixed and/or mobile node that provides a data serviceor a voice service and a receiver is a fixed and/or mobile node thatreceives a data service or a voice service. Therefore, a UE may serve asa transmitter and a BS may serve as a receiver, on an UpLink (UL).Likewise, the UE may serve as a receiver and the BS may serve as atransmitter, on a DownLink (DL).

The embodiments of the present disclosure may be supported by standardspecifications disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802.xx system, a 3^(rd) Generation Partnership Project (3GPP) system, a3GPP Long Term Evolution (LTE) system, and a 3GPP2 system. Inparticular, the embodiments of the present disclosure may be supportedby the standard specifications, 3GPP TS 36.211, 3GPP TS 36.212, 3GPP TS36.213, 3GPP TS 36.321 and 3GPP TS 36.331. That is, the steps or parts,which are not described to clearly reveal the technical idea of thepresent disclosure, in the embodiments of the present disclosure may beexplained by the above standard specifications. All terms used in theembodiments of the present disclosure may be explained by the standardspecifications.

Reference will now be made in detail to the embodiments of the presentdisclosure with reference to the accompanying drawings. The detaileddescription, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present disclosure, rather than to show the only embodiments thatcan be implemented according to the invention.

The following detailed description includes specific terms in order toprovide a thorough understanding of the present disclosure. However, itwill be apparent to those skilled in the art that the specific terms maybe replaced with other terms without departing the technical spirit andscope of the present disclosure.

For example, the term used in embodiments of the present disclosure, adata block is interchangeable with a transport block in the samemeaning. In addition, the MCS/TBS index table used in the LTE/LTE-Asystem can be defined as a first table or a legacy table, and theMCS/TBS index table which is used for supporting the 256QAM can bedefined as a second table or a new table.

The embodiments of the present disclosure can be applied to variouswireless access systems such as Code Division Multiple Access (CDMA),Frequency Division Multiple Access (FDMA), Time Division Multiple Access(TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), SingleCarrier Frequency Division Multiple Access (SC-FDMA), etc.

CDMA may be implemented as a radio technology such as UniversalTerrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented asa radio technology such as Global System for Mobile communications(GSM)/General packet Radio Service (GPRS)/Enhanced Data Rates for GSMEvolution (EDGE). OFDMA may be implemented as a radio technology such asIEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Evolved UTRA(E-UTRA), etc.

UTRA is a part of Universal Mobile Telecommunications System (UMTS).3GPP LTE is a part of Evolved UMTS (E-UMTS) using E-UTRA, adopting OFDMAfor DL and SC-FDMA for UL. LTE-Advanced (LTE-A) is an evolution of 3GPPLTE. While the embodiments of the present disclosure are described inthe context of a 3GPP LTE/LTE-A system in order to clarify the technicalfeatures of the present disclosure, the present disclosure is alsoapplicable to an IEEE 802.16e/m system, etc.

1. 3GPP LTE/LTE-A System

In a wireless access system, a UE receives information from an eNB on aDL and transmits information to the eNB on a UL. The informationtransmitted and received between the UE and the eNB includes generaldata information and various types of control information. There aremany physical channels according to the types/usages of informationtransmitted and received between the eNB and the UE.

1.1 System Overview

FIG. 1 illustrates physical channels and a general method using thephysical channels, which may be used in embodiments of the presentdisclosure.

When a UE is powered on or enters a new cell, the UE performs initialcell search (S11). The initial cell search involves acquisition ofsynchronization to an eNB. Specifically, the UE synchronizes its timingto the eNB and acquires information such as a cell Identifier (ID) byreceiving a Primary Synchronization Channel (P-SCH) and a SecondarySynchronization Channel (S-SCH) from the eNB.

Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast Channel (PBCH) from the eNB.

During the initial cell search, the UE may monitor a DL channel state byreceiving a Downlink Reference Signal (DL RS).

After the initial cell search, the UE may acquire more detailed systeminformation by receiving a Physical Downlink Control Channel (PDCCH) andreceiving a Physical Downlink Shared Channel (PDSCH) based oninformation of the PDCCH (S12).

To complete connection to the eNB, the UE may perform a random accessprocedure with the eNB (S13 to S16). In the random access procedure, theUE may transmit a preamble on a Physical Random Access Channel (PRACH)(S13) and may receive a PDCCH and a PDSCH associated with the PDCCH(S14). In the case of contention-based random access, the UE mayadditionally perform a contention resolution procedure includingtransmission of an additional PRACH (S15) and reception of a PDCCHsignal and a PDSCH signal corresponding to the PDCCH signal (S16).

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S17) and transmit a Physical Uplink Shared Channel (PUSCH)and/or a Physical Uplink Control Channel (PUCCH) to the eNB (S18), in ageneral UL/DL signal transmission procedure.

Control information that the UE transmits to the eNB is genericallycalled Uplink Control Information (UCI). The UCI includes a HybridAutomatic Repeat and reQuest Acknowledgement/Negative Acknowledgement(HARQ-ACK/NACK), a Scheduling Request (SR), a Channel Quality Indicator(CQI), a Precoding Matrix Index (PMI), a Rank Indicator (RI), etc.

In the LTE system, UCI is generally transmitted on a PUCCH periodically.However, if control information and traffic data should be transmittedsimultaneously, the control information and traffic data may betransmitted on a PUSCH. In addition, the UCI may be transmittedaperiodically on the PUSCH, upon receipt of a request/command from anetwork.

FIG. 2 illustrates exemplary radio frame structures used in embodimentsof the present disclosure.

FIG. 2(a) illustrates frame structure type 1. Frame structure type 1 isapplicable to both a full Frequency Division Duplex (FDD) system and ahalf FDD system.

One radio frame is 10 ms (T_(f)=307200·T_(s)) long, includingequal-sized 20 slots indexed from 0 to 19. Each slot is 0.5 ms(T_(slot)=15360·T_(s)) long. One subframe includes two successive slots.An i^(th) subframe includes 2i^(th) and (2i+1)^(th) slots. That is, aradio frame includes 10 subframes. A time required for transmitting onesubframe is defined as a Transmission Time Interval (TTI). T_(s) is asampling time given as T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸ (about 33 ns).One slot includes a plurality of Orthogonal Frequency DivisionMultiplexing (OFDM) symbols or SC-FDMA symbols in the time domain by aplurality of Resource Blocks (RBs) in the frequency domain.

A slot includes a plurality of OFDM symbols in the time domain SinceOFDMA is adopted for DL in the 3GPP LTE system, one OFDM symbolrepresents one symbol period. An OFDM symbol may be called an SC-FDMAsymbol or symbol period. An RB is a resource allocation unit including aplurality of contiguous subcarriers in one slot.

In a full FDD system, each of 10 subframes may be used simultaneouslyfor DL transmission and UL transmission during a 10-ms duration. The DLtransmission and the UL transmission are distinguished by frequency. Onthe other hand, a UE cannot perform transmission and receptionsimultaneously in a half FDD system.

The above radio frame structure is purely exemplary. Thus, the number ofsubframes in a radio frame, the number of slots in a subframe, and thenumber of OFDM symbols in a slot may be changed.

FIG. 2(b) illustrates frame structure type 2. Frame structure type 2 isapplied to a Time Division Duplex (TDD) system. One radio frame is 10 ms(T_(f)=307200·T_(s)) long, including two half-frames each having alength of 5 ms (=153600·T_(s)) long. Each half-frame includes fivesubframes each being 1 ms (=30720·T_(s)) long. An i^(th) subframeincludes 2i^(th) and (2i+1)^(th) slots each having a length of 0.5 ms(T_(slot)=15360·T_(s)). T_(s) is a sampling time given as T_(s)=1/(15kHz×2048)=3.2552×10⁻⁸ (about 33 ns).

A type-2 frame includes a special subframe having three fields, DownlinkPilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot(UpPTS). The DwPTS is used for initial cell search, synchronization, orchannel estimation at a UE, and the UpPTS is used for channel estimationand UL transmission synchronization with a UE at an eNB. The GP is usedto cancel UL interference between a UL and a DL, caused by themulti-path delay of a DL signal.

[Table 1] below lists special subframe configurations (DwPTS/GP/UpPTSlengths).

TABLE 1 Normal cyclic prefix in downlink Extended cyclic prefix indownlink UpPTS UpPTS Special Normal Extended Normal Extended subframecyclic prefix cyclic prefix cyclic prefix cyclic prefix configurationDwPTS in uplink in uplink DwPTS in uplink in uplink 0  6592 · T_(s) 2192· T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 1 19760 ·T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600· T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 · T_(s) 5  6592· T_(s) 4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 · T_(s) 23040 ·T_(s) 7 21952 · T_(s) — — — 8 24144 · T_(s) — — —

FIG. 3 illustrates an exemplary structure of a DL resource grid for theduration of one DL slot, which may be used in embodiments of the presentdisclosure.

Referring to FIG. 3, a DL slot includes a plurality of OFDM symbols inthe time domain. One DL slot includes 7 OFDM symbols in the time domainand an RB includes 12 subcarriers in the frequency domain, to which thepresent disclosure is not limited.

Each element of the resource grid is referred to as a Resource Element(RE). An RB includes 12×7 REs. The number of RBs in a DL slot, N_(DL)depends on a DL transmission bandwidth. A UL slot may have the samestructure as a DL slot.

FIG. 4 illustrates a structure of a UL subframe which may be used inembodiments of the present disclosure.

Referring to FIG. 4, a UL subframe may be divided into a control regionand a data region in the frequency domain. A PUCCH carrying UCI isallocated to the control region and a PUSCH carrying user data isallocated to the data region. To maintain a single carrier property, aUE does not transmit a PUCCH and a PUSCH simultaneously. A pair of RBsin a subframe are allocated to a PUCCH for a UE. The RBs of the RB pairoccupy different subcarriers in two slots. Thus it is said that the RBpair frequency-hops over a slot boundary.

FIG. 5 illustrates a structure of a DL subframe that may be used inembodiments of the present disclosure.

Referring to FIG. 5, up to three OFDM symbols of a DL subframe, startingfrom OFDM symbol 0 are used as a control region to which controlchannels are allocated and the other OFDM symbols of the DL subframe areused as a data region to which a PDSCH is allocated. DL control channelsdefined for the 3GPP LTE system include a Physical Control FormatIndicator Channel (PCFICH), a PDCCH, and a Physical Hybrid ARQ IndicatorChannel (PHICH).

The PCFICH is transmitted in the first OFDM symbol of a subframe,carrying information about the number of OFDM symbols used fortransmission of control channels (i.e. the size of the control region)in the subframe. The PHICH is a response channel to a UL transmission,delivering an HARQ ACK/NACK signal. Control information carried on thePDCCH is called Downlink Control Information (DCI). The DCI transportsUL resource assignment information, DL resource assignment information,or UL Transmission (Tx) power control commands for a UE group.

1.2 Physical Downlink Control Channel (PDCCH) 1.2.1 PDCCH Overview

The PDCCH may deliver information about resource allocation and atransport format for a Downlink Shared Channel (DL-SCH) (i.e. a DLgrant), information about resource allocation and a transport format foran Uplink Shared Channel (UL-SCH) (i.e. a UL grant), paging informationof a Paging Channel (PCH), system information on the DL-SCH, informationabout resource allocation for a higher-layer control message such as arandom access response transmitted on the PDSCH, a set of Tx powercontrol commands for individual UEs of a UE group, Voice Over InternetProtocol (VoIP) activation indication information, etc.

A plurality of PDCCHs may be transmitted in the control region. A UE maymonitor a plurality of PDCCHs. A PDCCH is transmitted in an aggregate ofone or more consecutive Control Channel Elements (CCEs). A PDCCH made upof one or more consecutive CCEs may be transmitted in the control regionafter subblock interleaving. A CCE is a logical allocation unit used toprovide a PDCCH at a code rate based on the state of a radio channel ACCE includes a plurality of RE Groups (REGs). The format of a PDCCH andthe number of available bits for the PDCCH are determined according tothe relationship between the number of CCEs and a code rate provided bythe CCEs.

1.2.2 PDCCH Structure

A plurality of PDCCHs for a plurality of UEs may be multiplexed andtransmitted in the control region. A PDCCH is made up of an aggregate ofone or more consecutive CCEs. A CCE is a unit of 9 REGs each REGincluding 4 REs. Four Quadrature Phase Shift Keying (QPSK) symbols aremapped to each REG. REs occupied by RSs are excluded from REGs. That is,the total number of REGs in an OFDM symbol may be changed depending onthe presence or absence of a cell-specific RS. The concept of an REG towhich four REs are mapped is also applicable to other DL controlchannels (e.g. the PCFICH or the PHICH). Let the number of REGs that arenot allocated to the PCFICH or the PHICH be denoted by N_(REG). Then thenumber of CCEs available to the system is N_(CCE(=└) ^(N) _(REG) ¹⁹_(┘)) and the CCEs are indexed from 0 to N_(CCE)−1.

To simplify the decoding process of a UE, a PDCCH format including nCCEs may start with a CCE having an index equal to a multiple of n. Thatis, given CCE i, the PDCCH format may start with a CCE satisfying

.

The eNB may configure a PDCCH with 1, 2, 4, or 8 CCEs. {1, 2, 4, 8} arecalled CCE aggregation levels. The number of CCEs used for transmissionof a PDCCH is determined according to a channel state by the eNB. Forexample, one CCE is sufficient for a PDCCH directed to a UE in a good DLchannel state (a UE near to the eNB). On the other hand, 8 CCEs may berequired for a PDCCH directed to a UE in a poor DL channel state (a UEat a cell edge) in order to ensure sufficient robustness.

[Table 2] below illustrates PDCCH formats. 4 PDCCH formats are supportedaccording to CCE aggregation levels as illustrated in [Table 2].

TABLE 2 PDCCH Number of Number of Number of format CCEs (n) REGs PDCCHbits 0 1 9 72 1 2 18 144 2 4 36 288 3 8 72 576

A different CCE aggregation level is allocated to each UE because theformat or Modulation and Coding Scheme (MCS) level of controlinformation delivered in a PDCCH for the UE is different. An MCS leveldefines a code rate used for data coding and a modulation order. Anadaptive MCS level is used for link adaptation. In general, three orfour MCS levels may be considered for control channels carrying controlinformation.

Regarding the formats of control information, control informationtransmitted on a PDCCH is called DCI. The configuration of informationin PDCCH payload may be changed depending on the DCI format. The PDCCHpayload is information bits. [Table 3] lists DCI according to DCIformats.

TABLE 3 DCI Format Description Format 0 Resource grants for the PUSCHtransmissions (uplink) Format 1 Resource assignments for single codewordPDSCH transmissions (transmission modes 1, 2 and 7) Format 1A Compactsignaling of resource assignments for single codeword PDSCH (all modes)Format 1B Compact resource assignments for PDSCH using rank-1 closedloop precoding (mode 6) Format 1C Very compact resource assignments forPDSCH (e.g. paging/broadcast system information) Format 1D Compactresource assignments for PDSCH using multi- user MIMO (mode 5) Format 2Resource assignments for PDSCH for closed-loop MIMO operation (mode 4)Format 2A Resource assignments for PDSCH for open-loop MIMO operation(mode 3) Format Power control commands for PUCCH and PUSCH with 3/3A2-bit/1-bit power adjustment Format 4 Scheduling of PUSCH in one UL cellwith multi-antenna port transmission mode

Referring to [Table 3], the DCI formats include Format 0 for PUSCHscheduling, Format 1 for single-codeword PDSCH scheduling, Format 1A forcompact single-codeword PDSCH scheduling, Format 1C for very compactDL-SCH scheduling, Format 2 for PDSCH scheduling in a closed-loopspatial multiplexing mode, Format 2A for PDSCH scheduling in anopen-loop spatial multiplexing mode, and Format 3/3A for transmission ofTransmission Power Control (TPC) commands for uplink channels. DCIFormat 1A is available for PDSCH scheduling irrespective of thetransmission mode of a UE.

The length of PDCCH payload may vary with DCI formats. In addition, thetype and length of PDCCH payload may be changed depending on compact ornon-compact scheduling or the transmission mode of a UE.

The transmission mode of a UE may be configured for DL data reception ona PDSCH at the UE. For example, DL data carried on a PDSCH includesscheduled data, a paging message, a random access response, broadcastinformation on a BCCH, etc. for a UE. The DL data of the PDSCH isrelated to a DCI format signaled through a PDCCH. The transmission modemay be configured semi-statically for the UE by higher-layer signaling(e.g. Radio Resource Control (RRC) signaling). The transmission mode maybe classified as single antenna transmission or multi-antennatransmission.

A transmission mode is configured for a UE semi-statically byhigher-layer signaling. For example, multi-antenna transmission schememay include transmit diversity, open-loop or closed-loop spatialmultiplexing, Multi-User Multiple Input Multiple Output (MU-MIMO), orbeamforming Transmit diversity increases transmission reliability bytransmitting the same data through multiple Tx antennas. Spatialmultiplexing enables high-speed data transmission without increasing asystem bandwidth by simultaneously transmitting different data throughmultiple Tx antennas. Beamforming is a technique of increasing theSignal to Interference plus Noise Ratio (SINR) of a signal by weightingmultiple antennas according to channel states.

A DCI format for a UE depends on the transmission mode of the UE. The UEhas a reference DCI format monitored according to the transmission modeconfigure for the UE. The following 10 transmission modes are availableto UEs:

(1) Transmission mode 1: Single antenna port (port 0);

(2) Transmission mode 2: Transmit diversity;

(3) Transmission mode 3: Open-loop spatial multiplexing when the numberof layer is larger than 1 or Transmit diversity when the rank is 1;

(4) Transmission mode 4: Closed-loop spatial multiplexing;

(5) Transmission mode 5: MU-MIMO;

(6) Transmission mode 6: Closed-loop rank-1 precoding;

(7) Transmission mode 7: Precoding supporting a single layertransmission, which does not based on a codebook (Rel-8);

(8) Transmission mode 8: Precoding supporting up to two layers, which donot based on a codebook (Rel-9);

(9) Transmission mode 9: Precoding supporting up to eight layers, whichdo not based on a codebook (Rel-10); and

(10) Transmission mode 10: Precoding supporting up to eight layers,which do not based on a codebook, used for CoMP (Rel-11).

1.2.3 PDCCH Transmission

The eNB determines a PDCCH format according to DCI that will betransmitted to the UE and adds a Cyclic Redundancy Check (CRC) to thecontrol information. The CRC is masked by a unique Identifier (ID) (e.g.a Radio Network Temporary Identifier (RNTI)) according to the owner orusage of the PDCCH. If the PDCCH is destined for a specific UE, the CRCmay be masked by a unique ID (e.g. a cell-RNTI (C-RNTI)) of the UE. Ifthe PDCCH carries a paging message, the CRC of the PDCCH may be maskedby a paging indicator ID (e.g. a Paging-RNTI (P-RNTI)). If the PDCCHcarries system information, particularly, a System Information Block(SIB), its CRC may be masked by a system information ID (e.g. a SystemInformation RNTI (SI-RNTI)). To indicate that the PDCCH carries a randomaccess response to a random access preamble transmitted by a UE, its CRCmay be masked by a Random Access-RNTI (RA-RNTI).

Then the eNB generates coded data by channel-encoding the CRC-addedcontrol information. The channel coding may be performed at a code ratecorresponding to an MCS level. The eNB rate-matches the coded dataaccording to a CCE aggregation level allocated to a PDCCH format andgenerates modulation symbols by modulating the coded data. Herein, amodulation order corresponding to the MCS level may be used for themodulation. The CCE aggregation level for the modulation symbols of aPDCCH may be one of 1, 2, 4, and 8. Subsequently, the eNB maps themodulation symbols to physical REs (i.e. CCE to RE mapping).

1.2.4 Blind Decoding (BD)

A plurality of PDCCHs may be transmitted in a subframe. That is, thecontrol region of a subframe includes a plurality of CCEs, CCE 0 to CCEN_(CCE,k)−1. N_(CCE,k) is the total number of CCEs in the control regionof a k^(th) subframe. A UE monitors a plurality of PDCCHs in everysubframe. This means that the UE attempts to decode each PDCCH accordingto a monitored PDCCH format.

The eNB does not provide the UE with information about the position of aPDCCH directed to the UE in an allocated control region of a subframe.Without knowledge of the position, CCE aggregation level, or DCI formatof its PDCCH, the UE searches for its PDCCH by monitoring a set of PDCCHcandidates in the subframe in order to receive a control channel fromthe eNB. This is called blind decoding. Blind decoding is the process ofdemasking a CRC part with a UE ID, checking a CRC error, and determiningwhether a corresponding PDCCH is a control channel directed to a UE bythe UE.

The UE monitors a PDCCH in every subframe to receive data transmitted tothe UE in an active mode. In a Discontinuous Reception (DRX) mode, theUE wakes up in a monitoring interval of every DRX cycle and monitors aPDCCH in a subframe corresponding to the monitoring interval. ThePDCCH-monitored subframe is called a non-DRX subframe.

To receive its PDCCH, the UE should blind-decode all CCEs of the controlregion of the non-DRX subframe. Without knowledge of a transmitted PDCCHformat, the UE should decode all PDCCHs with all possible CCEaggregation levels until the UE succeeds in blind-decoding a PDCCH inevery non-DRX subframe. Since the UE does not know the number of CCEsused for its PDCCH, the UE should attempt detection with all possibleCCE aggregation levels until the UE succeeds in blind decoding of aPDCCH.

In the LTE system, the concept of Search Space (SS) is defined for blinddecoding of a UE. An SS is a set of PDCCH candidates that a UE willmonitor. The SS may have a different size for each PDCCH format. Thereare two types of SSs, Common Search Space (CSS) andUE-specific/Dedicated Search Space (USS).

While all UEs may know the size of a CSS, a USS may be configured foreach individual UE. Accordingly, a UE should monitor both a CSS and aUSS to decode a PDCCH. As a consequence, the UE performs up to 44 blinddecodings in one subframe, except for blind decodings based on differentCRC values (e.g., C-RNTI, P-RNTI, SI-RNTI, and RA-RNTI).

In view of the constraints of an SS, the eNB may not secure CCEresources to transmit PDCCHs to all intended UEs in a given subframe.This situation occurs because the remaining resources except forallocated CCEs may not be included in an SS for a specific UE. Tominimize this obstacle that may continue in the next subframe, aUE-specific hopping sequence may apply to the starting point of a USS.

[Table 4] illustrates the sizes of CSSs and USSs.

TABLE 4 PDCCH Number of Number of candidates Number of candidates formatCCEs (n) in common search space in dedicated search space 0 1 — 6 1 2 —6 2 4 4 2 3 8 2 2

To mitigate the load of the UE caused by the number of blind decodingattempts, the UE does not search for all defined DCI formatssimultaneously. Specifically, the UE always searches for DCI Format 0and DCI Format 1A in a USS. Although DCI Format 0 and DCI Format 1A areof the same size, the UE may distinguish the DCI formats by a flag forformat0/format 1a differentiation included in a PDCCH. Other DCI formatsthan DCI Format 0 and DCI Format 1A, such as DCI Format 1, DCI Format1B, and DCI Format 2 may be required for the UE.

The UE may search for DCI Format 1A and DCI Format 1C in a CSS. The UEmay also be configured to search for DCI Format 3 or 3A in the CSS.Although DCI Format 3 and DCI Format 3A have the same size as DCI Format0 and DCI Format 1A, the UE may distinguish the DCI formats by a CRCscrambled with an ID other than a UE-specific ID.

An SS

is a PDCCH candidate set with a CCE aggregation level L∈{1, 2, 4, 8}.The CCEs of PDCCH candidate set m in the SS may be determined by thefollowing equation.

$\begin{matrix}{\mspace{79mu} \text{?}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack \\{\text{?}\text{indicates text missing or illegible when filed}} & \;\end{matrix}$

where

is the number of PDCCH candidates with CCE aggregation level L to bemonitored in the SS,

, i is the index of a CCE in each PDCCH candidate, and

.

where n_(s) is the index of a slot in a radio frame.

As described before, the UE monitors both the USS and the CSS to decodea PDCCH. The CSS supports PDCCHs with CCE aggregation levels {4, 8} andthe USS supports PDCCHs with CCE aggregation levels {1, 2, 4, 8}. [Table5] illustrates PDCCH candidates monitored by a UE.

TABLE 5 Search space S_(k) ^((L)) Number of PDCCH Type Aggregation levelL Size [in CCEs] candidates M^((L)) UE- 1 6 6 specific 2 12 6 4 8 2 8 162 Common 4 16 4 8 16 2

Referring to [Equation 1], for two aggregation levels, L=4 and L=8,Y_(k) is set to 0 in the CSS, whereas Y_(k) is defined by [Equation 2]for aggregation level L in the USS.

$\begin{matrix}{\mspace{79mu} \text{?}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack \\{\text{?}\text{indicates text missing or illegible when filed}} & \;\end{matrix}$

where

,

indicating an RNTI value. A=39827 and D=65537.

2. Carrier Aggregation (CA) Environment 2.1 CA Overview

A 3GPP LTE system (conforming to Rel-8 or Rel-9) (hereinafter, referredto as an LTE system) uses Multi-Carrier Modulation (MCM) in which asingle Component Carrier (CC) is divided into a plurality of bands. Incontrast, a 3GPP LTE-A system (hereinafter, referred to an LTE-A system)may use CA by aggregating one or more CCs to support a broader systembandwidth than the LTE system. The term CA is interchangeably used withcarrier combining, multi-CC environment, or multi-carrier environment.

In the present disclosure, multi-carrier means CA (or carrier combining)Herein, CA covers aggregation of contiguous carriers and aggregation ofnon-contiguous carriers. The number of aggregated CCs may be differentfor a DL and a UL. If the number of DL CCs is equal to the number of ULCCs, this is called symmetric aggregation. If the number of DL CCs isdifferent from the number of UL CCs, this is called asymmetricaggregation. The term CA is interchangeable with carrier combining,bandwidth aggregation, spectrum aggregation, etc.

The LTE-A system aims to support a bandwidth of up to 100 MHz byaggregating two or more CCs, that is, by CA. To guarantee backwardcompatibility with a legacy IMT system, each of one or more carriers,which has a smaller bandwidth than a target bandwidth, may be limited toa bandwidth used in the legacy system.

For example, the legacy 3GPP LTE system supports bandwidths {1.4, 3, 5,10, 15, and 20 MHz} and the 3GPP LTE-A system may support a broaderbandwidth than 20 MHz using these LTE bandwidths. A CA system of thepresent disclosure may support CA by defining a new bandwidthirrespective of the bandwidths used in the legacy system.

There are two types of CA, intra-band CA and inter-band CA. Intra-bandCA means that a plurality of DL CCs and/or UL CCs are successive oradjacent in frequency. In other words, the carrier frequencies of the DLCCs and/or UL CCs are positioned in the same band. On the other hand, anenvironment where CCs are far away from each other in frequency may becalled inter-band CA. In other words, the carrier frequencies of aplurality of DL CCs and/or UL CCs are positioned in different bands. Inthis case, a UE may use a plurality of Radio Frequency (RF) ends toconduct communication in a CA environment.

The LTE-A system adopts the concept of cell to manage radio resources.The above-described CA environment may be referred to as a multi-cellenvironment. A cell is defined as a pair of DL and UL CCs, although theUL resources are not mandatory. Accordingly, a cell may be configuredwith DL resources alone or DL and UL resources.

For example, if one serving cell is configured for a specific UE, the UEmay have one DL CC and one UL CC. If two or more serving cells areconfigured for the UE, the UE may have as many DL CCs as the number ofthe serving cells and as many UL CCs as or fewer UL CCs than the numberof the serving cells, or vice versa. That is, if a plurality of servingcells are configured for the UE, a CA environment using more UL CCs thanDL CCs may also be supported.

CA may be regarded as aggregation of two or more cells having differentcarrier frequencies (center frequencies). Herein, the term ‘cell’ shouldbe distinguished from ‘cell’ as a geographical area covered by an eNB.Hereinafter, intra-band CA is referred to as intra-band multi-cell andinter-band CA is referred to as inter-band multi-cell.

In the LTE-A system, a Primacy Cell (PCell) and a Secondary Cell (SCell)are defined. A PCell and an SCell may be used as serving cells. For a UEin RRC_CONNECTED state, if CA is not configured for the UE or the UEdoes not support CA, a single serving cell including only a PCell existsfor the UE. On the contrary, if the UE is in RRC_CONNECTED state and CAis configured for the UE, one or more serving cells may exist for theUE, including a PCell and one or more SCells.

Serving cells (PCell and SCell) may be configured by an RRC parameter. Aphysical-layer ID of a cell, PhysCellId is an integer value ranging from0 to 503. A short ID of an SCell, SCellIndex is an integer value rangingfrom 1 to 7. A short ID of a serving cell (PCell or SCell),ServeCellIndex is an integer value ranging from 1 to 7. IfServeCellIndex is 0, this indicates a PCell and the values ofServeCellIndex for SCells are pre-assigned. That is, the smallest cellID (or cell index) of ServeCellIndex indicates a PCell.

A PCell refers to a cell operating in a primary frequency (or a primaryCC). A UE may use a PCell for initial connection establishment orconnection reestablishment. The PCell may be a cell indicated duringhandover. In addition, the PCell is a cell responsible forcontrol-related communication among serving cells configured in a CAenvironment. That is, PUCCH allocation and transmission for the UE maytake place only in the PCell. In addition, the UE may use only the PCellin acquiring system information or changing a monitoring procedure. AnEvolved Universal Terrestrial Radio Access Network (E-UTRAN) may changeonly a PCell for a handover procedure by a higher-layerRRCConnectionReconfiguraiton message including mobilityControlInfo to aUE supporting CA.

An SCell may refer to a cell operating in a secondary frequency (or asecondary CC). Although only one PCell is allocated to a specific UE,one or more SCells may be allocated to the UE. An SCell may beconfigured after RRC connection establishment and may be used to provideadditional radio resources. There is no PUCCH in cells other than aPCell, that is, in SCells among serving cells configured in the CAenvironment.

When the E-UTRAN adds an SCell to a UE supporting CA, the E-UTRAN maytransmit all system information related to operations of related cellsin RRC_CONNECTED state to the UE by dedicated signaling. Changing systeminformation may be controlled by releasing and adding a related SCell.Herein, a higher-layer RRCConnectionReconfiguration message may be used.The E-UTRAN may transmit a dedicated signal having a different parameterfor each cell rather than it broadcasts in a related SCell.

After an initial security activation procedure starts, the E-UTRAN mayconfigure a network including one or more SCells by adding the SCells toa PCell initially configured during a connection establishmentprocedure. In the CA environment, each of a PCell and an SCell mayoperate as a CC. Hereinbelow, a Primary CC (PCC) and a PCell may be usedin the same meaning and a Secondary CC (SCC) and an SCell may be used inthe same meaning in embodiments of the present disclosure.

FIG. 6 illustrates an example of CCs and CA in the LTE-A system, whichare used in embodiments of the present disclosure.

FIG. 6(a) illustrates a single carrier structure in the LTE system.There are a DL CC and a UL CC and one CC may have a frequency range of20 MHz.

FIG. 6(b) illustrates a CA structure in the LTE-A system. In theillustrated case of FIG. 6(b), three CCs each having 20 MHz areaggregated. While three DL CCs and three UL CCs are configured, thenumbers of DL CCs and UL CCs are not limited. In CA, a UE may monitorthree CCs simultaneously, receive a DL signal/DL data in the three CCs,and transmit a UL signal/UL data in the three CCs.

If a specific cell manages N DL CCs, the network may allocate M (M≦N) DLCCs to a UE. The UE may monitor only the M DL CCs and receive a DLsignal in the M DL CCs. The network may prioritize L (L≦M≦N) DL CCs andallocate a main DL CC to the UE. In this case, the UE should monitor theL DL CCs. The same thing may apply to UL transmission.

The linkage between the carrier frequencies of DL resources (or DL CCs)and the carrier frequencies of UL resources (or UL CCs) may be indicatedby a higher-layer message such as an RRC message or by systeminformation. For example, a set of DL resources and UL resources may beconfigured based on linkage indicated by System Information Block Type 2(SIB2). Specifically, DL-UL linkage may refer to a mapping relationshipbetween a DL CC carrying a PDCCH with a UL grant and a UL CC using theUL grant, or a mapping relationship between a DL CC (or a UL CC)carrying HARQ data and a UL CC (or a DL CC) carrying an HARQ ACK/NACKsignal.

2.2 Cross Carrier Scheduling

Two scheduling schemes, self-scheduling and cross carrier scheduling aredefined for a CA system, from the perspective of carriers or servingcells. Cross carrier scheduling may be called cross CC scheduling orcross cell scheduling.

In self-scheduling, a PDCCH (carrying a DL grant) and a PDSCH aretransmitted in the same DL CC or a PUSCH is transmitted in a UL CClinked to a DL CC in which a PDCCH (carrying a UL grant) is received.

In cross carrier scheduling, a PDCCH (carrying a DL grant) and a PDSCHare transmitted in different DL CCs or a PUSCH is transmitted in a UL CCother than a UL CC linked to a DL CC in which a PDCCH (carrying a ULgrant) is received.

Cross carrier scheduling may be activated or deactivated UE-specificallyand indicated to each UE semi-statically by higher-layer signaling (e.g.RRC signaling).

If cross carrier scheduling is activated, a Carrier Indicator Field(CIF) is required in a PDCCH to indicate a DL/UL CC in which aPDSCH/PUSCH indicated by the PDCCH is to be transmitted. For example,the PDCCH may allocate PDSCH resources or PUSCH resources to one of aplurality of CCs by the CIF. That is, when a PDCCH of a DL CC allocatesPDSCH or PUSCH resources to one of aggregated DL/UL CCs, a CIF is set inthe PDCCH. In this case, the DCI formats of LTE Release-8 may beextended according to the CIF. The CIF may be fixed to three bits andthe position of the CIF may be fixed irrespective of a DCI format size.In addition, the LTE Release-8 PDCCH structure (the same coding andresource mapping based on the same CCEs) may be reused.

On the other hand, if a PDCCH transmitted in a DL CC allocates PDSCHresources of the same DL CC or allocates PUSCH resources in a single ULCC linked to the DL CC, a CIF is not set in the PDCCH. In this case, theLTE Release-8 PDCCH structure (the same coding and resource mappingbased on the same CCEs) may be used.

If cross carrier scheduling is available, a UE needs to monitor aplurality of PDCCHs for DCI in the control region of a monitoring CCaccording to the transmission mode and/or bandwidth of each CC.Accordingly, an appropriate SS configuration and PDCCH monitoring areneeded for the purpose.

In the CA system, a UE DL CC set is a set of DL CCs scheduled for a UEto receive a PDSCH, and a UE UL CC set is a set of UL CCs scheduled fora UE to transmit a PUSCH. A PDCCH monitoring set is a set of one or moreDL CCs in which a PDCCH is monitored. The PDCCH monitoring set may beidentical to the UE DL CC set or may be a subset of the UE DL CC set.The PDCCH monitoring set may include at least one of the DL CCs of theUE DL CC set. Or the PDCCH monitoring set may be defined irrespective ofthe UE DL CC set. DL CCs included in the PDCCH monitoring set may beconfigured to always enable self-scheduling for UL CCs linked to the DLCCs. The UE DL CC set, the UE UL CC set, and the PDCCH monitoring setmay be configured UE-specifically, UE group-specifically, orcell-specifically.

If cross carrier scheduling is deactivated, this implies that the PDCCHmonitoring set is always identical to the UE DL CC set. In this case,there is no need for signaling the PDCCH monitoring set. However, ifcross carrier scheduling is activated, the PDCCH monitoring set may bedefined within the UE DL CC set. That is, the eNB transmits a PDCCH onlyin the PDCCH monitoring set to schedule a PDSCH or PUSCH for the UE.

FIG. 7 illustrates a cross carrier-scheduled subframe structure in theLTE-A system, which is used in embodiments of the present disclosure.

Referring to FIG. 7, three DL CCs are aggregated for a DL subframe forLTE-A UEs. DL CC ‘A’ is configured as a PDCCH monitoring DL CC. If a CIFis not used, each DL CC may deliver a PDCCH that schedules a PDSCH inthe same DL CC without a CIF. On the other hand, if the CIF is used byhigher-layer signaling, only DL CC ‘A’ may carry a PDCCH that schedulesa PDSCH in the same DL CC ‘A’ or another CC. Herein, no PDCCH istransmitted in DL CC ‘B’ and DL CC ‘C’ that are not configured as PDCCHmonitoring DL CCs.

FIG. 8 is conceptual diagram illustrating a construction of servingcells according to cross-carrier scheduling.

Referring to FIG. 8, an eNB (or BS) and/or UEs for use in a radio accesssystem supporting carrier aggregation (CA) may include one or moreserving cells. In FIG. 8, the eNB can support a total of four servingcells (cells A, B, C and D). It is assumed that UE A may include Cells(A, B, C), UE B may include Cells (B, C, D), and UE C may include CellB. In this case, at least one of cells of each UE may be composed of PCell. In this case, P Cell is always activated, and S Cell may beactivated or deactivated by the eNB and/or UE.

The cells shown in FIG. 8 may be configured per UE. The above-mentionedcells selected from among cells of the eNB, cell addition may be appliedto carrier aggregation (CA) on the basis of a measurement report messagereceived from the UE. The configured cell may reserve resources forACK/NACK message transmission in association with PDSCH signaltransmission. The activated cell is configured to actually transmit aPDSCH signal and/or a PUSCH signal from among the configured cells, andis configured to transmit CSI reporting and Sounding Reference Signal(SRS) transmission. The deactivated cell is configured not totransmit/receive PDSCH/PUSCH signals by an eNB command or a timeroperation, and CRS reporting and SRS transmission are interrupted.

3. Sounding Reference Signal (SRS) 3.1 SRS in LTE/LTE-A System

FIG. 9 illustrates one of methods for transmitting SRS used atembodiments of the present invention.

An SRS is used for channel quality estimation to enablefrequency-selective scheduling on uplink. At this time, SRS transmissionis performed regardless of uplink data transmission and/or uplinkcontrol information transmission. The SRS may be used for the purpose ofenhancing power control or supporting various start-up functions for UEsnot recently scheduled. For example, the various start-up functionsinclude initial modulation and coding scheme (MCS) selection, initialpower control for data transmission, timing advance (TA), and so-calledfrequency semi-selective scheduling. At this time, frequencysemi-selective scheduling means that the frequency resource is assignedselectively for the first slot of a subframe and hops pseudorandomly toa different frequency in the second slot.

In addition, the SRS can be used for downlink channel quality estimationunder the assumption that the wireless channel is reciprocal betweenuplink and downlink. This assumption is especially valid in a timedivision duplex (TDD) system where the uplink and downlink share thesame frequency spectrum and are separated in the time domain.

The subframes in which SRSs are transmitted by any UE within the cellare indicated by cell-specific broadcast signaling. A 4-bitcell-specific ‘srsSubframeConfiguration’ parameter indicates 15 possiblesets of subframes in which an SRS may be transmitted within each radioframe. This configurability provides flexibility in adjusting the SRSoverhead depending on deployment scenario. A 16^(th) configurationswitches the SRS off completely in the cell, which may for example beappropriate for a cell serving primarily high-speed UEs.

The SRS transmissions are always performed in the last SC-FDMA symbol inthe configured subframes. Thus, the SRS and DM RS are located indifferent SC-FDMA symbols. PUSCH data transmission is not permitted onthe SC-FDMA symbol designated for SRS, resulting in a worst-casesounding overhead of up to 7% in every subframe.

Each SRS symbol is generated by basis sequences where for a given timeinstance and bandwidth all the UEs in a cell use the same basis sequencewhile SRS transmissions from multiple UEs in the same time and band in acell are distinguished orthogonally by different cyclic shifts of thebasis sequence assigned to different UEs. SRS sequences from differentcells can be distinguished by assigning different basis sequences indifferent cells where orthogonality is not guaranteed between differentbasis sequences.

3.2 Method for UE to Transmit Sounding Signal

Hereinafter, a description will be given of methods for a UE to transmitan SRS.

A UE may transmit an SRS on an SRS resource per serving cell based ontwo trigger types. Trigger type 0 means a periodic SRS transmissionmethod indicated by higher layer signaling and trigger type 1 means aperiodic SRS transmission method requested by DCI format 0/4/1Atransmitted through PDCCH for FDD and TDD schemes or DCI format 2B/2C/2Dtransmitted through PDCCH for the TDD scheme.

If both of the SRS transmission according to the trigger type 0 and theSRS transmission according to the trigger type 1 occurs at the samesubframe in the same serving cell, the UE performs only the SRStransmission according to the trigger type 1. The user equipment may beassigned SRS parameters for the trigger type 0 and/or trigger type 1 ineach serving cell. Hereinafter, a description will be given of the SRSparameters, which are configured serving-cell-specifically orsemi-statically for the trigger type 0 and/or trigger type 1 by a higherlayer signal.

The transmission comb, k _(TC) defined in clause 5.5.3.2 of 3GPP TS36.211 is configured for the trigger type 0 and each configuration ofthe trigger type 1, respectively.

The starting physical resource block assignment parameter, n_(RRC)defined in clause 5.5.3.2 of 3GPP TS 36.211 is configured for thetrigger type 0 and each configuration of the trigger type 1,respectively.

A duration parameter for the trigger type 0 may be configured for asingle subframe. Alternatively, the duration parameter may beindefinitely configured until it is released.

A srs-ConfigIndex I_(SRS) parameter indicating an SRS transmissionperiod, T_(SRS) and an SRS subframe offset, T_(offset) for the triggertype 0 is defined in Table 7 and Table 8 below. A srs-ConfigIndexparameter, I_(SRS) indicating an SRS transmission period, T_(SRS,1) andan SRS subframe offset, T_(offset,1) for the trigger type 1 is definedin Table 10 and Table 11 below.

The SRS bandwidth parameter, B_(SRS) defined in 5.5.3.2 of 3GPP TS36.211 is configured for the trigger type 0 and each configuration ofthe trigger type 1, respectively.

The frequency hopping bandwidth parameter, b_(hop) defined in 5.5.3.2 of3GPP TS 36.211 is configured for the trigger type 0.

The cyclic shift parameter, n_(SRS) ^(cs) defined in 3GPP TS 36.211 isconfigured for the trigger type 0 and each configuration of the triggertype 1.

An antenna port number parameter, N_(p) is configured for the triggertype 0 and each configuration of the trigger type 1.

For the trigger type 1 and DCI format 4, three sets of SRS parameters(e.g., srs-ConfigApDCI-Format4) is configured by a higher layer signal.2-bit of an SRS request field contained in the DCI format 4 indicates anSRS parameter set shown in Table 6 below.

TABLE 6 Value of SRS request filed Description ‘00’ No type 1 SRStrigger ‘01’ The 1^(st) SRS parameter set configured by higher layers‘10’ The 2^(nd) SRS parameter set configured by higher layers ‘11’ The3^(rd) SRS parameter set configured by higher layers

For the trigger type 1 and DCI format 0, one SRS parameter set,srs-ConfigApCDI-Format0 is configured by higher layer signaling. For thetrigger type 1 and DCI format 1A/2B/2C/2D, one common SRS parameter set,srs-ConfigApCDI-Format1a2b2c is configured by higher layer signaling.

If 1 bit of an SRS request field contained in the DCI format0/1A/2B/2C/2D is set to ‘1’, the trigger type 1 can be triggered (i.e.,positive SRS request). If the UE is assigned the SRS parameters for theDCI format 0/1A/2B/2C/2D through higher layer signaling, 1 bit of theSRS request field is included in the DCI format 0/1A with respect toframe structure type 1 and 1 bit of the SRS request field is included inthe DCI format 0/1A/2B/2C/2D with respect to the frame structure type 2.

A serving-cell-specific SRS transmission band C_(SRS) andserving-cell-specific SRS transmission subframes are configured byhigher layer signaling (e.g., MAC message, RRC message, etc.).

If a UE supporting transmit antenna selection is allowed (or activated)to select an antenna in a given serving cell, an index of the UE antennafor transmitting SRS during a time n_(SRS) is determined according toEquation 3 or Equation 4.

a(n _(SRS))=n _(SRS) mod 2   [Equation 3]

Equation 3 shows a UE antenna index in case that frequency hopping isdeactivated in some or all of a sounding bandwidth (i.e.,b_(hop)≧B_(SRS)).

$\begin{matrix}{{a\left( n_{SRS} \right)} = \left\{ {\begin{matrix}{\begin{pmatrix}{n_{SRS} + \left\lfloor {n_{SRS}/2} \right\rfloor +} \\{\beta \cdot \left\lfloor {n_{SRS}/K} \right\rfloor}\end{pmatrix}{mod}{\mspace{11mu} \;}2} & {{when}\mspace{14mu} K{\; \mspace{11mu}}{is}\mspace{14mu} {even}} \\{n_{SRS}{mod}{\mspace{11mu} \;}2} & {{when}\mspace{14mu} K{\mspace{11mu} \;}{is}\mspace{14mu} {odd}}\end{matrix},\mspace{79mu} {\beta = \left\{ \begin{matrix}1 & {{{where}\mspace{14mu} K\mspace{14mu} {mod}\mspace{14mu} 4} = 0} \\0 & {otherwise}\end{matrix} \right.}} \right.} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Equation 4 shows a UE antenna index in case that frequency hopping isactivated (i.e., b_(hop)<B_(SRS)). The parameter values B_(SRS),b_(hop), N_(b), and n_(SRS) of Equation 3 and Equation 4, which areincorporated herein by reference, can be found in clause 5.5.3.2 of 3GPPTS 36.211. Except for the case where single SRS transmission is assignedto the UE, K is set to

$K = {\prod\limits_{b^{\prime} = b_{hop}}^{B_{SRS}}\; {N_{b^{\prime}}.}}$

In this case, it is assumed that N_(b) _(hop) =1 regardless of a valueof N_(b). If the UE is connected to at one or more serving cells, the UEis not expected to transmit SRS on different antenna portssimultaneously.

The UE may be configured to transmit SRS on N_(p) antenna ports of aserving cell where N_(p) may be informed the UE through a higher layersignal. In case of PUSCH transmission mode 1, N_(p) is set to N_(p)∈{0,1, 2, 4}. In case of PUSCH transmission mode 2 with two antenna portsconfigured for PUSCH, N_(p) is set to N_(p)∈{0, 1, 2}. In case of fourantenna ports configured for PUSCH, N_(p) is set to N_(p)∈{0, 1, 4}.

In case of a UE configured to transmit SRS on multiple antenna ports ofthe serving cell, the UE should transmit SRS for all the configuredtransmit antenna ports within one SC-FDMA symbol of the same subframe ofthe corresponding serving cell. The SRS transmission bandwidth andstarting physical resource block assignment parameters are the same forall the configured antenna ports of the corresponding serving cell.

In case of a UE not configured with multiple TAGs (timing advancedgroup), the UE does not transmit SRS whenever SRS transmission and PUSCHtransmission overlap each other in the same symbol. Here, TAG means agroup of serving cells with the same TA, which is used for matchinguplink synchronization with an eNB in a carrier aggregation (CA)environment.

In the case of TDD, if there is one SC-FDMA symbol in UpPTS of a givenserving cell, the SC-FDMA symbol can be used for SRS transmission. Ifthere are two SC-FDMA symbols in UpPTS of a given serving cell, the twoSC-FDMA symbols may be assigned to the same UE and both of them can beused for SRS transmission.

When trigger type 0 SRS transmission and PUCCH format 2/2a/2btransmission collide with each other in the same subframe, the UE notconfigured with the multiple TAGs does not perform the trigger type 0SRS transmission. When trigger type 1 SRS transmission and PUCCH format2a/2b transmission or PUCCH format 2 transmission for HARQ informationtransmission collide with each other in the same subframe, the UE notconfigured with the multiple TAGs does not perform the trigger type 1SRS transmission. When PUCCH format 2 transmission of which the purposeis not to transmit HARQ information and the trigger type 1 SRStransmission collide with each other in the same subframe, the UE notconfigured with the multiple TAGs does not perform the PUCCH format 2transmission.

In case that an ackNackSRS-SimultaneousTransmission parameter is set to‘FALSE’, if SRS transmission, PUCCH transmission for HARQ-ACKinformation transmission, and/or positive SR collide with each other inthe same subframe, the UE not configured with the multiple TAGs does notperform the SRS transmission. In case that theackNackSRS-SimultaneousTransmission parameter is set to ‘TRUE’, if theSRS transmission, the PUCCH transmission for the HARQ-ACK informationtransmission, and/or a shortened format of positive SR collide with eachother in the same subframe, the UE not configured with the multiple TAGsperforms the SRS transmission.

If the SRS transmission, PUCCH transmission for the HARQ informationtransmission, and/or a common PUCCH format of positive SR collide witheach other in the same subframe, the UE not configured with the multipleTAGs does not perform the SRS transmission.

If an interval for the SRS transmission overlaps with a PRACH region forpreamble format 4 in UpPTS or the interval exceeds the range of anuplink system bandwidth configured in the serving cell, the UE does notperform the SRS transmission.

Whether the UE simultaneously transmits PUCCH carrying HARQ-ACKinformation and SRS in the same subframe is determined based on theackNackSRS-SimultaneousTransmission parameter provided by a higherlayer. If the UE is configured to transmit the PUCCH carrying theHARQ-ACK information and the SRS in the same subframe, the UE transmitsthe HARQ-ACK and SR in cell-specific SRS subframes of a primary cell byusing a shortened PUCCH format. In this case, the HARQ-ACK or an SRsymbol corresponding to a location of the SRS is punctured. Even if theUE does not transmit SRS in a cell-specific SRS subframe of the primarycell, the shortened PUCCH format is used in the corresponding subframe.Otherwise, the UE uses common PUCCH format 1/1a/1b or common PUCCHformat 3 in order to transmit the HARQ-ACK and SR.

Table 7 and Table 8 show trigger type 0 of an SRS configuration withrespect to an SRS transmission periodicity parameter, T_(SRS) and an SRSsubframe offset parameter, T_(offset) defined in FDD and TDD,respectively.

TABLE 7 SRS Configuration SRS Periodicity SRS Subframe Index I_(SRS)T_(SRS) (ms) Offset T_(offset) 0-1 2 I_(SRS) 2-6 5 I_(SRS) − 2   7-16 10I_(SRS) − 7  17-36 20 I_(SRS) − 17 37-76 40 I_(SRS) − 37  77-156 80I_(SRS) − 77 157-316 160  I_(SRS) − 157 317-636 320  I_(SRS) − 317 637-1023 reserved reserved

TABLE 8 SRS Configuration SRS Periodicity SRS Subframe Index I_(SRS)T_(SRS) (ms) Offset T_(offset) 0 2 0, 1 1 2 0, 2 2 2 1, 2 3 2 0, 3 4 21, 3 5 2 0, 4 6 2 1, 4 7 2 2, 3 8 2 2, 4 9 2 3, 4 10-14 5 I_(SRS) − 1015-24 10 I_(SRS) − 15 25-44 20 I_(SRS) − 25 45-84 40 I_(SRS) − 45 85-164 80 I_(SRS) − 85 165-324 160  I_(SRS) − 165 325-644 320  I_(SRS)− 325  645-1023 reserved reserved

The SRS transmission periodicity parameter T_(SRS) is aserving-cell-specific value and is selected from a set of {2, 5, 10, 20,40, 80, 160, 320} ms or subframes. In case of the periodicity parameterT_(SRS) set to 2 ms in the TDD, two SRS resources are configured in ahalf frame including UL subframes in a given serving cell.

In case of T_(SRS)>2 in the TDD or FDD, the trigger type 0 of SRStransmission instances are determined as subframes satisfying thecondition of (10·n_(f)+k_(SRS)−T_(offset))modT_(SRS)=0 in a givenserving cell. Here, in the case of FDD, k_(SRS)={0, 1, . . . 9} means anindex of a subframe in a frame and in the case of TDD, k_(SRS) isdefined as shown in Table 9 below. Moreover, in case of T_(SRS)=2 inTDD, SRS transmission instances are determined as subframe satisfyingthe condition of (k_(SRS)=T_(offset))mod5=0.

TABLE 9 subframe index n 1 6 1st symbol 2nd symbol 1st symbol 2nd symbol0 of UpPTS of UpPTS 2 3 4 5 of UpPTS of UpPTS 7 8 9 k_(SRS) in case 0 12 3 4 5 6 7 8 9 UpPTS length of 2 symbols k_(SRS) in case 1 2 3 4 6 7 89 UpPTS length of 1 symbol

Table 10 and Table 11 show SRS transmission periodicity, T_(SRS,1) andSRS subframe offset, T_(offset,1) defined in FDD and TDD, respectively,in the case of trigger type 1 of SRS transmission.

TABLE 10 SRS Configuration SRS Periodicity SRS Subframe Index I_(SRS)T_(SRS, 1) (ms) Offset T_(offset, 1) 0-1 2 I_(SRS) 2-6 5 I_(SRS) − 2 7-16 10 I_(SRS) − 7 17-31 reserved reserved

TABLE 11 SRS Configuration SRS Periodicity SRS Subframe Index I_(SRS)T_(SRS, 1) (ms) Offset T_(offset, 1) 0 2 0, 1 1 2 0, 2 2 2 1, 2 3 2 0, 34 2 1, 3 5 2 0, 4 6 2 1, 4 7 2 2, 3 8 2 2, 4 9 2 3, 4 10-14 5 I_(SRS) −10 15-24 10 I_(SRS) − 15 25-31 reserved reserved

The periodicity parameter for SRS transmission, T_(SRS,1) is aserving-cell-specific value and is selected from a set of {2, 5, 10} msor subframes. In case that the SRS transmission periodicity is set to 2ms in the TDD, two SRS resources are configured in a half frameincluding UL subframes in a given serving cell.

In case of a UE configured with type 1 SRS transmission in serving cellc and not configured with a carrier indicator filed, if the UE detects apositive SRS request from PDCCH/EPDCCH for scheduling PUSCH/PDSCH, theUE transmits SRS in serving cell c.

In case of a UE configured with type 1 SRS transmission in serving cellc and configured with a carrier indicator field, if the UE detects apositive SRS request from PDCCH/EPDCCH for scheduling PUSCH/PDSCH, theUE transmits SRS in serving cell c corresponding to the carrierindicator filed.

If a UE configured with type 1 SRS transmission in serving cell cdetects a positive SRS request from subframe n of serving cell c, incase of T_(SRS,1)>2 in TDD, the UE initiates SRS transmission in a firstsubframe satisfying the conditions of n+k,k≧4 and(10·n_(f)+k_(SRS)−T_(offset,1))modT_(SRS,1)=0. Alternatively, in case ofT_(SRS,1)=2 in TDD, the UE initiates SRS transmission in a firstsubframe satisfying the condition of (k_(SRS)−T_(offset,1))mod5=0. Here,in the case of FDD, k_(SRS){0, 1, . . . , 9} means a subframe index offrame n_(f).

A UE configured with trigger type 1 SRS transmission is not expected toreceive a type 1 SRS triggering event related to trigger type 1 SRStransmission parameters, which are configured with different values withrespect to the same serving cell and the same subframe by higher layersignaling.

If SRS transmission collides with retransmission of the same transportblock or PUSCH transmission corresponding to a random access response aspart of a contention-based random access procedure, a UE does nottransmit SRS.

3.3 Periodic SRS Transmission and Aperiodic SRS Transmission

FIG. 10(a) is a diagram showing the concept of periodic SRS transmissionand FIG. 10(b) is a diagram showing the concept of aperiodic SRStransmission.

First, periodic SRS transmission will be described. Referring to FIG.10(a), SRS transmission parameters for SRS transmission are transmittedfrom an eNB to a UE via a higher layer signal (e.g., an RRC signal)(S1010).

The SRS transmission parameters may include an SRS transmissionbandwidth parameter indicating bandwidth occupied by one SRStransmission, a hopping bandwidth parameter indicating a frequencyregion in which SRS transmission hops to a frequency, a frequencyposition parameter indicating a position where SRS transmission startsin the frequency region, a transmission comb parameter indicating an SRStransmission position or pattern, a cyclic shift parameter fordistinguishing between SRSs, a period parameter indicating an SRStransmission period and a subframe offset parameter indicating asubframe in which an SRS is transmitted. At this time, the subframeoffset parameter may indicate a cell-specific SRS subframe or aUE-specific SRS subframe.

The UE may periodically perform SRS transmission at a time interval of 2ms to 160 ms based on the SRS transmission parameters (S1030).

At this time, since SRS symbols cannot be used for PUSCH transmission,all UEs within the cell may previously know in which subframe SRStransmission is performed in the cell.

Next, aperiodic SRS transmission will be described. Aperiodic SRStransmission is triggered through signaling on a PDCCH as part ofscheduling grant. The frequency region structure of aperiodic SRStransmission is equal to that of periodic SRS transmission. However,when an aperiodic SRS is transmitted is determined per UE via higherlayer signaling.

Referring to FIG. 10(b), SRS transmission parameters for SRStransmission are transmitted from an eNB to a UE via a higher layersignal (e.g., an RRC signal) (S1020).

At this time, the SRS transmission parameters used for aperiodic SRStransmission are basically equal to those used for periodic SRStransmission.

The eNB transmits a PDCCH signal or an E-PDCCH signal having an SRSrequest field to the UE when aperiodic SRS transmission is requested. Atthis time, the E-PDCCH signal means control information transmitted viaa PDSCH region. In addition, for the description of the PDCCH signal,refer to Chapter 1 (S1040).

The UE, which has explicitly received the request for aperiodic SRStransmission in step S1140, may perform aperiodic SRS transmission inthe subframe (S1060).

4. SRS Transmission Method for MTC UE 4.1 MTC UE

In the LTE-A system, implementation of a low-price/low-specificationuser equipment focusing on data communication such as reading of ameter, measurement of a water level, utilization of a monitoring camera,stock management of a vending machine, and the like is considered forthe next generation wireless communication system. In the embodiments ofthe present invention, such a low-price/low-specification user equipmentis called a machine type communication (MTC) user equipment forconvenience.

In case of an MTC UE, since the amount of transmitted data is relativelysmall and uplink/downlink data transmission and reception is performedoccasionally, it is efficient to lower a price of the MTC UE and toreduce battery consumption according to the low data transmission rate.Such an MTC UE has a characteristic of low mobility and thus its channelenvironment is rarely changed. In the current LTE-A, it has beenconsidered to allow the MTC UE to have a coverage wider than theprevious one. To this end, various techniques for coverage enhancementhave been also discussed.

The MTC UE may be installed in an area (e.g., a basement, etc.) withpoor transmission conditions compared to the legacy UE (i.e., normalUE). For such MTC UEs, a relay node can be installed but the cost ofinvestment in facilities may be too high. Therefore, it is effective toprovide stable communication to the MTC UE operating in an area withpoor radio conditions by repeatedly transmitting a downlink or uplinkchannel

Hereinafter, SRS transmission methods for the MTC UE will be describedin detail. The following SRS transmission methods can be operated basedon the methods described in clauses 1 to 3.

4.2 SRS Transmission Method-1

SRS is transmitted on an uplink channel to support uplink channelmeasurement at an eNB and then it is used to perform scheduling forPUSCH. In this case, a channel environment where an MTC UE is placed mayhave poor radio conditions. Thus, the MTC UE may be configured torepeatedly transmit the SRS to enable the eNB to perform channelestimation efficiently.

4.2.1 SRS Configuration Method for MTC UE

SRS from an MTC UE can be configured to be repeatedly transmitted.Transmission of an SRS sequence is determined by cell-specificparameters and UE-specific parameters. In this case, it is preferred toperform the repeated SRS transmission in a time domain. To this end, asequence characteristic and a transmission band of the SRS can beconfigured consistently during an interval for transmitting the SRSrepeatedly.

The SRS transmission band is determined based on an SRS bandwidth and anSRS hopping related parameter. Each of the SRS bandwidth and SRS hoppingrelated parameter may be configured to maintain a constant value duringthe SRS repeat transmission interval.

The SRS sequence can be determined by (1) a sequence group number u, (2)a base sequence number v determined according to a transmission band andpresence of sequence hopping, (3) a cyclic shift parameter correspondingto an SRS transmission parameter, and (4) an SRS transmit antenna portas defined in 3GPP TS 36.211.

The reason for the repeated SRS transmission in the time domain is toimprove performance of the uplink channel estimation at the eNB throughcombining SRSs, which are received by the eNB. Thus, it is preferredthat there is no change in the SRS sequence during the repeattransmission interval. In other words, the same SRS sequences can berepeatedly transmitted.

To this end, the parameter values u and v, which are used in determiningthe SRS sequence, are preferred to be fixed in the repeat transmissioninterval. In addition, sequence group hopping for the SRS may bedeactivated or each of the parameter values u and v may be set to aconstant value in the repeat transmission interval.

Hereinafter, a description will be given of a method for configuring thesequence group number u. The parameter value u for the repeated SRStransmission can be defined according to Equation 5 below.

u=(f _(gh)(n _(p))+f _(ss))mod 30   [Equation 5]

In Equation 5, f_(gh) is a function indicating a group hopping patternand f_(ss) indicates an SRS sequence shift pattern. In this case, f_(ss)is determined as a value satisfying the condition of f_(ss)=f_(ss)^(PUCCH)=N_(ID) ^(cell)mod 30. N_(ID) ^(cell) means a cell identifierand n_(p) indicates an SRS transmission period. That is, in this case,the same SRS sequence is used in all SRS repeat transmission intervals.If the sequence group hopping is disabled, f_(gh)(n_(p)) may be set to 0(i. e., f_(gh)(n_(p))=0).

As another method, if it is configured that sequence group hopping ismaintained consistently during a first SRS repeat transmission intervaland a different SRS sequence is used in a next SRS repeat transmissioninterval, f_(gh)(n_(p)), which is function of an SRS transmission period(n_(p)), may have a random value. In other words, the UE may transmitthe SRS using different SRS sequences every SRS repeat transmissionperiod. However, even in this case, the same SRS sequence is repeatedlytransmitted in one SRS repeat transmission period.

Hereinafter, a description will be given of a method for configuring abase sequence number v. In case that the SRS transmission band is equalto or less than 6 RB, the value v can be set to ‘0’ similar to that ofthe conventional LTE/LTE-A system. If the SRS transmission band is equalto or greater than 6 RB, the value v can also be set to ‘0’ bydeactivating the sequence hopping. That is, according to theconventional LTE/LTE-A system, when the sequence hopping is activated,the v has different values. On the other hand, in case that the MTC UEtransmits the SRS repeatedly, the v has the same value.

Alternatively, similar to a sequence group hoping method, the SRSsequence hopping can be configured to have a constant v value in onerepeat transmission interval and have a different v value in a nextrepeat transmission interval. In this case, the value v may set thefunction of the SRS transmission period (n_(p)) to have a random value.

As described in the methods, the UE can generate and transmit the SRSaccording to parameter values configured by the eNB. In this case, theUE may generate the same SRS sequence and repeatedly transmit the samesequence in all SRS transmission intervals. Alternatively, the UE maygenerate a different SRS sequence and repeatedly transmit the differentSRS sequence in each SRS transmission interval.

4.2.2 SRS Transmission Method

In an MTC environment, a transmission bandwidth of an MTC UE may berestricted to a specific bandwidth (e.g., 6 PRB). In this case, atransmission bandwidth of SRS to be transmitted by the UE may also berestricted. However, since a system bandwidth may be greater than thetransmission bandwidth supported by the MTC UE, the SRS may beconfigured to be transmitted through a subband in order for the MTC UEto be scheduled with the subband. For instance, the system bandwidth maybe divided into a plurality of subbands corresponding to the bandwidthof the MTC UE and the SRS may be configured to be transmitted through atleast one of the corresponding subbands.

In this case, the order of SRS transmission through the subband may beconfigured according to the order of subband indices (e.g., frequencydescending order of subband indices) or a predetermined order. Thus, theUE may transmit the SRS through the subband in the above order. When theSRS is transmitted through at least one of the subbands, it is preferredthat after completion of the repeated SRS transmission in one subband,the repeated SRS transmission is initiated in a next subband.

To perform the repeated SRS transmission in the time domain, the UEneeds to be configured with a plurality of SRS configurations. Forexample, it is preferred that an eNB configures a UE to repeatedlytransmit SRS in SRS subframes commonly configured in a serving cell.Particularly, the SRS subframes commonly configured in the serving cellmay be (1) cell-specific SRS subframes, which are assigned to a generalUE in the conventional LTE/LTE-A system, for SRS transmission or (2) newcell-specific SRS subframes defined for an MTC UE. In other words, acell-specific MTC SRS subframe can be defined for the repeated SRStransmission of the MTC UE.

The following methods can be considered for the repeated SRStransmission.

(1) Method 1: The eNB can explicitly inform the UE of subframes to beused in the repeated SRS transmission among the cell-specific SRSsubframes. In this case, the repeated SRS transmission may be performedonly in the subframes indicated by the eNB and normal SRS transmissionmay be performed in the rest of the subframes. That is, the repeated SRStransmission may not be continuously performed in each SRS transmissionperiod.

(2) Method 2: The eNB can indicate the number of subframes required forthe repeated transmission starting from a UE-specific SRS subframeoffset among the cell-specific SRS subframes. In the embodiments of thepresent invention, the UE-specific SRS subframes can be considered to beincluded in the cell-specific SRS subframes.

According to the method 2, the repeated SRS transmission can becontinuously performed only in UE-specific SRS subframes indicated bythe eNB among the cell-specific SRS subframes. For instance, the SRStransmission is performed only in the subframes indicated by the eNB andthe repeated SRS transmission may not be performed in the remaining SRStransmission period.

(3) Method 3: The eNB can explicitly indicate a first subframe number(i.e., subframe offset) of a UE-specific SRS subframe for performing therepeated SRS transmission and a last SRS subframe among thecell-specific SRS subframes.

According to the method 3, the UE can repeatedly transmit the SRS in thecell-specific SRS subframes in the range of from the first subframecorresponding to the SRS subframe offset to the last SRS subframeindicated by the eNB. Depending on a cell-specific SRS subframeconfiguration, the repeated SRS transmission may not be performed onconsecutive SRS subframes. For instance, when the cell-specific SRSsubframes are not configured in a consecutive manner, the SRS may berepeatedly transmitted only in cell-specific SRS subframes contained inan SRS repeat transmission period and not be repeatedly transmitted inthe remaining subframes.

FIG. 11 is a diagram illustrating one example of a method for an MTCuser equipment to repeatedly transmit SRS in case of a trigger type 0 ofSRS transmission schemes. In particular, FIG. 11 (a) shows a method fora normal UE to transmit SRS periodically and FIG. 11 (b) shows a methodfor an MTC UE to transmit SRS periodically.

FIG. 12 is a diagram illustrating one example of a method for an MTCuser equipment to repeatedly transmit SRS in case of a trigger type 1 ofSRS transmission schemes. In particular, FIG. 12 (a) shows a method fora normal UE to transmit SRS aperiodically and FIG. 12 (b) shows a methodfor an MTC UE to transmit SRS aperiodically.

The SRS transmission method described in clause 3 can be referred forthe SRS transmission methods in FIG. 11 (a) and FIG. 12 (a). Moreover,the methods 1 to 3 explained in clause 4.2.2 can be applied to the SRStransmission methods for the MTC UE in FIG. 11 (b) and FIG. 12 (b). Ineach of FIG. 11 (b) and FIG. 12 (b), the MTC UE may generate SRSaccording to the SRS configuration method described in clause 4.2.1 andthen repeatedly transmit the generated SRS a predetermined number oftimes in each SRS transmission period or in a subframe where an SRStransmission request is received.

In this case, the SRS may be repeatedly transmitted only in SRSsubframes indicated by an eNB among cell-specific SRS subframes (method1). Alternatively, the SRS may be repeated transmitted only in SRSsubframes indicated by an eNB among UE-specific SRS subframes ofcell-specific SRS subframes (method 2 or method 3).

In this case, if the cell-specific SRS subframes are not configured in aconsecutive manner, the MTC UE may repeatedly transmit the SRS only incell-specific SRS subframes within the predetermined number of subframesreserved for the repeated SRS transmission (1). Alternatively, the MTCUE may repeatedly transmit the SRS in cell-specific SRS subframes of afirst SRS repeat transmission interval. However, if the MTC UE fails torepeatedly transmit the SRS the predetermined number of times, the MTCUE may repeatedly transmit the SRS the remaining number of times incell-specific SRS subframes of a next SRS repeat transmission interval(2).

Alternatively, the eNB may set the number of repeated SRS transmissiontimes and the SRS repeat transmission interval. For instance, it isassumed that the MTC UE needs to perform n times of the repeated SRStransmission in an MTC environment. When setting the SRS repeattransmission interval, the eNB may set the SRS repeat transmissioninterval in consideration of the number of repeated transmission timesn, the number of cell-specific SRS subframes x, and the number ofUE-specific SRS subframes y. In case of y>x>=n, the eNB may instruct theUE to perform n times of the repeated SRS transmission only. In case ofy>n>x or x>n>y, the eNB may instruct the UE to increase a repeattransmission period as much as the number of repeated transmission times(n−x) or (n−y).

4.3 SRS Transmission Method-2

While transmitting SRS repeatedly, a UE may use different SRStransmission combs to facilitate multiplexing with SRS from another UE.

That is, there are two SRS transmission combs in a single RB. Forrepeated SRS transmission, an MTC UE may be configured to use a firstSRS transmission comb in a repeat transmission interval and use a secondSRS transmission comb for SRS transmitted in a different subframe. Tothis end, an eNB may inform an SRS transmission comb configurationthrough higher layer signaling/MAC signaling/L1 signaling.

4.4 SRS Transmission Restriction

Hereinafter, a description will be given of SRS transmission restrictionmethods applicable to the above-mentioned SRS transmission methods.

4.4.1 SRS Transmission Restriction According to Trigger Type

SRS transmission can be divided into trigger type 0 (i.e., periodic SRStransmission) in which transmission is performed according to aconfiguration of a higher layer and trigger type 1 (i.e., aperiodic SRStransmission) in which initiation of transmission is indicated by PDCCH.In this case, since an MTC UE is generally placed in poor MTC radioconditions, the MTC UE may be configured to support only one mode of thetrigger type 0 and trigger type 1.

For instance, the MTC UE can be configured to support only the triggertype 1. In this case, only if there is a request from an eNB, the MTC UEcan perform repeated SRS transmission. Of course, the MTC UE can beconfigured to support only the trigger type 0. When the trigger type 0is supported only, the MTC UE may assume that there is no aperiodic SRSrequest from the eNB. In this case, an SRS request field, which isincluded in a DCI format, for the aperiodic SRS request may not betransmitted or it may be used for other purposes.

4.4.2 Operations in Simultaneous Transmission of SRS and Other UplinkChannels

When an MTC UE transmits SRS repeatedly, there may be a situation inwhich repeated SRS transmission needs to be performed in a subframereserved for uplink control information transmission (e.g., HARQ-ACKtransmission, SR (scheduling request) transmission, periodic CSItransmission and/or aperiodic CSI transmission, etc.). In this case, aneNB may enable the SRS transmission not to be performed in thecorresponding subframe by using a higher layer signal (e.g., RRC signal,MMC signal, etc.) in advance. In other words, in case that an SRS repeattransmission interval overlaps with the subframe in which thetransmission of the uplink control information is performed, therepeated SRS transmission may not be performed in the overlappingsubframe.

In this case, even though the SRS is not actually transmitted, the eNBand/or UE may count the number of repeated transmission times byconsidering it as that the SRS transmission is performed. If the SRS isfrequently dropped due to the simultaneous transmission of the SRS andother uplink channels, each UE has a different SRS repeat transmissionperiod and it will increase complexity in multiplexing. However,according to the above method, the problem of increased complexity inmultiplexing can be solved. In other words, although the MTC UE fails torepeatedly transmit the SRS a prescribed number of times until a nextrepeat transmission time due to the SRS drop after setting an SRS repeattransmission start time, the MTC UE and/or eNB can initiate new repeatedSRS transmission based on SRS configuration information or SRSconfiguration parameters. This method has advantages of facilitatingcontrol by the eNB and reducing complexity of the system.

As another embodiment, the MTC UE can complete the repeated SRStransmission by counting the number of repeated times for actual SRStransmission. According to this embodiment, although complexity in SRSmultiplexing is increased, channel estimation performance based on SRScan be enhanced.

4.4.3 Transmission Format Restriction Method

In case that there are subframes where repeated SRS transmission andrepeated HARQ-ACK/SR transmission needs to be performed simultaneously,an MTC UE may be configured to perform the HARQ-ACK/SR transmission incell-specific SRS subframes using a shortened format. Particularly,ackNackSRS-SimultaneousTransmission parameter for a normal UE of theLTE/LTE-A system and MTCackNackSRS-SimultaneousTransmission parameterfor an MTC UE can be considered as examples. For example, in case thatthe ackNackSRS-SimultaneousTransmission parameter and theMTCackNackSRS-SimultaneousTransmission parameter are set to ‘TRUE’ and‘FALSE’, respectively, an SRS transmission format in each of alegacy-cell-specific SRS subframe and an MTC-cell-specific SRS subframecan be determined according to configurations of the two parameters. Asanother example, instead of the two above-mentioned parameters, otherparameters defined in the LTE/LTE-A system can be used. In this case,the SRS transmission format is configured in the form of the shortenedformat at all times.

However, for the HARQ-ACK/SR transmission, the MTC UE can be configuredto use a common format in subframes except the cell-specific SRSsubframes during an SRS repeat transmission interval. In this case, aneNB (i.e., receiving end) for receiving HARQ-ACK/SR may separatelycombine the HARQ-ACK/SR transmitted in the cell-specific SRS subframeand the HARQ-ACK/SR transmitted in the subframe except the cell-specificSRS subframe and then perform final decoding.

4.5 Method for Using DM-RS

In the above-mentioned embodiments of the present invention, it may beconfigured that DM-RS in a different subframe is used to improveperformance of channel estimation with respect to UL data transmittedthrough PUSCH.

To this end, it is preferable that the DM-RS in the different subframeis configured to have the same frequency band and the same sequence,similar to a method of configuring parameters for repeated SRStransmission. That is, it is preferred that an eNB transmits to a UE ahigher layer parameter indicating whether the DM-RS in the differentsubframe is used for channel estimation.

For instance, the eNB may transmit to an MTC UE information indicating aspecific subframe of which DM-RS is used together with repeated SRStransmission to estimate an uplink channel

Hereinafter, a description will be given of a method of setting a uvalue of a DM-RS sequence.

The eNB can deactivate group hopping for determining u of PUSCH DM-RSused for uplink channel estimation together with the repeated SRStransmission. This is because to have the same DM-RS sequence duringrepeated transmission of the PUSCH DM-RS. Alternatively, the eNB may setthe u value of the PUSCH DM-RS to be maintained as the same value in afirst subframe set and to have a different value in a second subframeset. The u value of the DM-RS can be represented as Equation 6 below.

u=(f _(gh)(n _(p) ^(DMRS))+f _(ss))mod 30   [Equation 6]

In Equation 6, n_(p) ^(DMRS) is a parameter indicating the number ofsubframes in which UL channel estimation is performed by using theDM-RS, f_(ss)=f_(ss) ^(PU SCH)=(f_(ss) ^(PU CCH)+Δ_(ss)), and Δ_(ss)∈{0,1, . . . , 29}. In this case, if DM-RSs of p consecutive subframes areused, n_(p) ^(DMRS) may mean a parameter for assigning a new randomvalue to each of the p subframes.

Hereinafter, a description will be given of a method of setting a vvalue of a DM-RS sequence. If a DM-RS transmission band is equal to orless than 6 RB, it is preferred to set the v value to ‘0’ in the samemanner as the conventional one. If the DM-RS transmission band is equalto or greater than 6 RB, it is preferred to set the v value to ‘0’ bydeactivating sequence hopping.

Alternatively, similar to sequence group hopping, sequence hopping maybe configured to have the same v value in a subframe set where channelestimation is performed using the DM-RS and have a different v value ina next transmission interval. In this case, the v value may be set to arandom value resulting from a function of the parameter n_(p) ^(DMRS)indicating the number of subframes in which the channel estimation isperformed using the DM-RS. That is, each subframe set in which thechannel estimation is performed using the DM-RS may be configured tohave a different v value.

6. Apparatuses

Apparatuses illustrated in FIG. 13 are means that can implement themethods described before with reference to FIGS. 1 to 12.

A UE may act as a transmitter on a UL and as a receiver on a DL. An eNBmay act as a receiver on a UL and as a transmitter on a DL.

That is, each of the UE and the eNB may include a Transmission (Tx)module 1340 or 1350 and a Reception (Rx) module 1360 or 1370, forcontrolling transmission and reception of information, data, and/ormessages, and an antenna 1300 or 1310 for transmitting and receivinginformation, data, and/or messages.

Each of the UE and the eNB may further include a processor 1320 or 1330for implementing the afore-described embodiments of the presentdisclosure and a memory 1380 or 1390 for temporarily or permanentlystoring operations of the processor 1320 or 1330.

The embodiments of the present invention can be implemented using theabove-described components and functions of the UE and the eNB. Forexample, the processor of the eNB may previously allocate an uplinkchannel region for SRS transmission between small cells by combining themethods disclosed above in clauses 1 to 5. In addition, the processor ofthe eNB may control the TX module to transmit resource allocationinformation on the allocated channel region to the UE through a higherlayer signal in an explicit manner Moreover, the processor of the UE maygenerate SRS based on an SRS transmission parameter received through thehigher layer signal and then transmit the generated SRS through thechannel region indicated by the SRS transmission parameter. Furtherdetails can be found in clauses 1 to 5.

The Tx and Rx modules of the UE and the eNB may perform a packetmodulation/demodulation function for data transmission, a high-speedpacket channel coding function, OFDMA packet scheduling, TDD packetscheduling, and/or channelization. Each of the UE and the eNB of FIG. 13may further include a low-power Radio Frequency (RF)/IntermediateFrequency (IF) module.

Meanwhile, the UE may be any of a Personal Digital Assistant (PDA), acellular phone, a Personal Communication Service (PCS) phone, a GlobalSystem for Mobile (GSM) phone, a Wideband Code Division Multiple Access(WCDMA) phone, a Mobile Broadband System (MBS) phone, a hand-held PC, alaptop PC, a smart phone, a Multi Mode-Multi Band (MM-MB) terminal, etc.

The smart phone is a terminal taking the advantages of both a mobilephone and a PDA. It incorporates the functions of a PDA, that is,scheduling and data communications such as fax transmission andreception and Internet connection into a mobile phone. The MB-MMterminal refers to a terminal which has a multi-modem chip built thereinand which can operate in any of a mobile Internet system and othermobile communication systems (e.g. CDMA 2000, WCDMA, etc.).

Embodiments of the present disclosure may be achieved by various means,for example, hardware, firmware, software, or a combination thereof.

In a hardware configuration, the methods according to exemplaryembodiments of the present disclosure may be achieved by one or moreApplication Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, the methods according to theembodiments of the present disclosure may be implemented in the form ofa module, a procedure, a function, etc. performing the above-describedfunctions or operations. A software code may be stored in the memory1380 or 1390 and executed by the processor 1320 or 1330. The memory islocated at the interior or exterior of the processor and may transmitand receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present disclosure maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent disclosure. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentdisclosure or included as a new claim by a subsequent amendment afterthe application is filed.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to various wireless access systemsincluding a 3GPP system, a 3GPP2 system, and/or an IEEE 802.xx system.Besides these wireless access systems, the embodiments of the presentdisclosure are applicable to all technical fields in which the wirelessaccess systems find their applications.

What is claimed is:
 1. A method of transmitting an sounding referencesignal (SRS) by a user equipment (UE) in a wireless access systemsupporting machine type communication (MTC), the method comprising:receiving an SRS transmission parameter configured for repeated SRStransmission from an evolved Node B (eNB); and performing the repeatedSRS transmission during a predetermined SRS repeat transmission intervalaccording to the SRS transmission parameter, wherein if the SRS repeattransmission interval overlaps with a subframe where an uplink controlinformation is transmitted, the repeated SRS transmission is notperformed in the overlapping subframes.
 2. The method of claim 1,wherein the repeated transmission of the SRS is periodically performedaccording to a prescribed SRS transmission period.
 3. The method ofclaim 1, wherein the repeated transmission of the SRS is aperiodicallyperformed only if there is a request from the eNB.
 4. The method ofclaim 1, wherein the repeated transmission of the SRS is performed inonly cell-specific SRS subframes.
 5. The method of claim 4, furthercomprising: receiving indications of subframes in which the repeated SRStransmission will be performed among the cell-specific SRS subframesfrom the eNB, wherein the repeated transmission of the SRS is performedin only the indicated subframes.
 6. The method of claim 1, wherein theSRS transmission parameter comprises a parameter for generating an SRSsequence for the repeated SRS transmission and wherein the SRS parameteris configured such that the SRS sequence has an identical sequenceduring the prescribed SRS repeat transmission interval.
 7. A userequipment (UE) for transmitting an sounding reference signal (SRS) in awireless access system supporting machine type communication (MTC),comprising: a receiver; a transmitter; and a processor for supportingthe SRS transmission, wherein the processor is configured to control thereceiver to receive an SRS transmission parameter configured forrepeated SRS transmission from an evolved Node B (eNB) and control thetransmitter to perform the repeated SRS transmission during apredetermined SRS repeat transmission interval according to the SRStransmission parameter and wherein if the SRS repeat transmissioninterval overlaps with a subframe where an uplink control information istransmitted, the repeated SRS transmission is not performed in theoverlapping subframes.
 8. The UE of claim 7, wherein the repeatedtransmission of the SRS is periodically performed according to aprescribed SRS transmission period.
 9. The UE of claim 7, wherein therepeated transmission of the SRS is aperiodically performed only ifthere is a request from the eNB.
 10. The UE of claim 7, wherein therepeated transmission of the SRS is performed in only cell-specific SRSsubframes.
 11. The UE of claim 10, wherein the processor is furtherconfigured to control the receiver to receive indications of subframesin which the repeated SRS transmission will be performed among thecell-specific SRS subframes from the eNB and wherein the repeatedtransmission of the SRS is performed in only the indicated subframes.12. The UE of claim 7, wherein the SRS transmission parameter comprisesa parameter for generating an SRS sequence for the repeated SRStransmission and wherein the SRS parameter is configured such that theSRS sequence has an identical sequence during the prescribed SRS repeattransmission interval.